Fire detection systems are often installed within commercial, residential, educational, or governmental buildings, to list a few examples. These fire detection systems typically include control panels and fire detection devices, which monitor the buildings for indicators of fire. In one example, the fire detection devices are individually addressable smoke detectors that are part of a network. Other examples include networks of stand-alone detectors with no control panel.
One common type of fire detection devices are photoelectric (or optical) smoke detectors. The optical smoke detectors often include a baffle system, which defines a detection chamber, to block ambient light while also allowing air to flow into the detection chamber. The optical smoke detectors further include a smoke detection system within the detection chamber for detecting the presence of smoke. The smoke detection system typically comprises a chamber light source and a scattered light photodetector. When smoke fills the detection chamber it causes the light from the chamber light source to be scattered within the chamber and detected by the scattered light photodetector. When no smoke or other scatter medium is present, the photodetector only receives a small background signal from the light source.
In many systems, the fire detection devices send event data, characterizing the level of detected scatter light for example, to the control panel. There the event data is analyzed. The panel will cause an alarm if the smoke exceeds a threshold, for example. In other examples, this analysis is performed on the detector itself, or a hybrid of on-detector and on-panel analysis.
As air flows through the detection chamber over time, dirt and dust can accumulate inside and around the detection chamber. This is especially true for fire detection devices installed in harsh environments such as kitchens or rooms with cigarette smoke. Additionally, it is not uncommon for insects or spiders to build nests or webs in or on the detectors. Even in devices installed in environments that are not considered harsh (such as offices), dirt and dust gradually accumulate inside the detection chamber. Typically, as dirt or dust accumulates inside the detection chamber, the background signal level increases.
Currently, building codes require that the fire detection systems be tested annually. This annual testing is performed because these fire detection devices have a number of different failure modes. For example, the electronics or optics of the device can fail. Likewise, the devices can become so dirty that the baffle systems become clogged. Additionally, it is not uncommon for the fire detection devices to get painted over.
The annual testing of the fire detection devices is commonly performed by a technician performing a walkthrough test. The technician walks through the building and manually tests each of the fire detection devices of the fire detection system. In the case of smoke detectors, the technician often uses a special testing device including an artificial smoke generating apparatus housed within a hood at the end of a pole. The technician places the hood over the fire detection device and the artificial smoke generating device releases artificial smoke near the detector. If the smoke detector is functioning properly, it will trigger in response to the smoke. The technician repeats this process for every smoke detector of the fire alarm system.
On the other hand, self-testing fire detection devices have been proposed. In one specific example, a self-test circuit for a smoke detector periodically tests whether the sensitivity of a scattered light photodetector is within a predetermined range of acceptable sensitivities. If the sensitivity of the scattered light photodetector is out of the predetermined range of acceptable sensitivities, then a fault indication is produced.